Many methods for manufacturing heat exchange tubes exist in the art. Most of the methods involve folding a coated sheet of metal to form channels, applying a flux material to the folded metal, and then heating the folded metal and flux material while applying a brazing material. During the heating process, the flux material cleans the surfaces of the metal so that the brazing material can readily flow into any gaps between the folds to seal the gap and form joints. Generally, the entire sheet of metal is coated with the brazing material and then later with flux or the entire sheet is coated with both the brazing and flux materials. Some metals or metal alloys, such as aluminum, are cladded with a more readily brazable metal or alloy prior to use to facilitate this type of manufacturing process.
Some examples of this technology can be found in the patent literature. European Patent Application No. 0 302 232 discloses a heat exchange tube wherein the terminal edges of the sheet material are rolled towards the center of the material past vertical so that the edges are parallel with the sheet material when they are brazed thereto. U.S. Pat. No. 4,633,056 discloses a method for manufacturing a heat exchange tube having an oval cross-section and a cross web for such tubes. The tube is joined using electron beam welding. U.S. Pat. No. 5,186,251 discloses a heat exchange tube with double row flow passages. U.S. Pat. No. 5,441,106 discloses a heat exchange tube that includes a plurality of internal fins that extend along the length of the tube. The tube is formed of cladded aluminum billet and brazed together. U.S. Pat. No. 5,579,837 discloses a heat exchange tube having a partition formed by two legs having an angle of about 7° to 15° between them. The entire tube is coated with brazing flux prior to brazing. U.S. Pat. No. 5,704,423 discloses a heat exchange tube produced by assembling a main portion and a secondary portion of two different pieces of metal, each generally aluminum or an aluminum alloy. This patent does not disclose a process of brazing that is free of flux. U.S. Pat. No. 5,765,634 discloses a heat exchange tube divided in two by a reinforcing partition. The partition consists of a pleat extending into the interior of the tube and formed in the sheet metal strip from which the tube is fabricated. Prior to joining the ends of the metal strip, one face of the strip is coated with braze metal. This patent also does not disclose a process of brazing that is free of flux. As can be seen from the foregoing, no one shape is universally accepted for radiator tube manufacture. These references would generally use a flux or paste containing flux to braze together the metal, especially when using aluminum.
U.S. Pat. No. 5,837,388 discloses an aluminum solder material for use in a brazing process for an aluminum alloy heat exchanger. U.S. Pat. No. 6,156,134 discloses a bonding method for stainless steal parts that can be used in a heat exchanger. These patents do not disclose the use of a plasma or wire arc to thermally spray the brazing material onto the heat exchanger.
The flux material is generally corrosive to the tube material and the furnace used in the brazing process. This use of flux adds expense to the process. It is often desirable to manufacture a heat exchange tube without the application of a flux. One such fluxless copper alloy used as a brazing filler material is described in U.S. Pat. No. 5,378,294. It may be useful to use such an alloy as a brazing filler material in the manufacture of heat exchanger tubing.
Another method of forming heat transfer tubes is described in the inventor's prior U.S. Pat. No. 6,530,514. This patent describes and claims fluxless brazing techniques generally and brazing with paste and foil specifically.
Fluxless brazing materials can be applied to metal surfaces by a variety of coating techniques. Specifically, one method of depositing brazing material includes a paint-like slurry made by mixing brazing powder with binders and solvents that are used as carriers. After the deposition, the carriers are evaporated by heat and forced air leaving a powder adhered to the surface by the binder. Foil may also be inserted between the materials to be brazed, although the foil may not stick to the surface of flat tubes. The drying step increases the time necessary for this technique and reduces the production rate in such processes.
When using the paint-like slurry the density of the coating using powder is lower than a solid layer since the powder is typically spherical in shape and air may become entrapped between the spheres. The thickness of the coating must often be increased in order to increase the amount of braze material on the coated surface. Increased thickness is not a viable option when a close fit is required in assembly of some parts. The thickness may also cause an increase in part size which may not be desirable in some applications.
Despite these known coating techniques and the prior knowledge of manufacture of heat transfer devices, there still is a need for improved heat exchanger device formation processes, and the present invention discloses a preferred process which avoids the disadvantages of the known techniques.